Fix pressing keys on a hardware keyboard showing the software keyboard

[Susko3]

• Closes osu!lazer android keyboard keep showing after pressing keys in keyboard osu#17561
• Closes Keyboard called at startup osu#6282
  • can't repro, but not returning null and forcing OnCheckIsTextEditor() => true is the probable cause

By default, Android will show the software keyboard while text input is active even if a hardware keyboard is connected. And since we were always returning a valid IInputConnection, Android would "conveniently" show the software keyboard when any hardware keys are pressed.

Pressing keys will (re)focus the current view and probe OnCreateInputConnection() to see if the input method is to be activated.

The docs specify to return null when input methods are not supported by a specific View. Since we have only one View, returning null here means to disable text input for the time being (until a textbox is focused and AndroidTextInput is activated).

[Susko3]

Discovered a bug, this has completely broken AndroidInputConnection, as evident by CommitText not getting fired.

[peppy]

Probably best to get a second check on this one, looks okay code-wise though.